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while ( lines_to_read() ) {
    if ( the_line_is_the_right_header()) {
        if ( the_line_also_has_impressions) {
...
        }
    }
}

while (files_to_read()) {
     next unless the_line_is_the_right_header();
     while ( lines_to_read() ) {
...
         last;    # no need to look further in this file
     }
}